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Putting soybean aphid scouting on “speed dial”

Time is money, and soybean farmers and crop consultants don’t seem to have a surplus of either these days. So some Extension service experts have begun recommending a faster, simpler method of scouting for soybean aphids.

While crop advisers typically collect extensive insect data on randomly selected plants in a field, the idea behind speed scouting is to get in, count up to a pre-determined number of aphids on selected plants and make a decision to spray or not spray.

“The premise of speed scouting is you look at a plant, and, if you count 40 aphids or more, the plant is infested,” says Erin Hodgson, Extension entomologist with Iowa State University. “If you have 39 or less, the plant is not infested. It saves time because when you get to 40, you can stop counting and move to the next plant.”

Using conventional methods, crop scouts will look at 30 or so plants and calculate an average number of aphids per plant to determine if the insects have reached an economic treatment threshold.

“Speed scouting is only going to tell you if you should treat or not treat,” says Hodgson, who developed this sampling plan as a graduate student at the University of Minnesota before joining Iowa State University May 1. “When you have plants that are uniformly, heavily infested, this is where it’s going to save you some time.”

Soybean aphids have become a major pest for soybean growers in several Midwest and some Midsouth states since they were first discovered in Wisconsin in 2000. (The insects are native to eastern Asia but arrived in the U.S. via an accidental introduction.)

Outbreaks occurred in Iowa in 2003, 2005, 2007 and 2008 with infestations rising to several thousand insects per plant when left untreated. Nearly 3 million acres of soybeans were sprayed with insecticides in Iowa in 2003 and about half the acres in 2007. In 2004, only 50,000 acres were treated because of very low aphid populations.

“Some people like it; some people don’t; some people have questions about it; and others just haven’t heard about it,” says Hodgson. “What I’m attempting to do is to make speed scouting understandable for everyone.”

The cooler temperatures that have occurred in much of the Midwest in July can actually work to the benefit of the soybean aphid, says Matt O’Neal, assistant professor of entomology at Iowa State and a speaker at the ISU-ISA tour.

“When temperatures are in the 70-80° range, you can actually get a doubling of the aphid populations every 5.5 to seven days,” he said, noting that when temperatures go above the 80° mark, the developmental time is lengthened. Above 95°, the lifespan is shortened, and no young are produced.

Entomologists generally use a treatment threshold of 250 aphids/soybean plant across the Midwest. Although economic damage normally occurs at higher levels, this threshold provides a five- to seven-day lead time to schedule an insecticide application to protect yield.

Directions for speed scouting include:

Go to a plant at random and start counting aphids. If less than 40 aphids are on the entire plant, mark a minus for that non-infested plant. If you reach 40 aphids, stop counting and mark a plus for that plant.

Walk 30 paces or paces at random to find the next plant. Repeat Step 1 until 11 plants are sampled in different areas of the field. Total the number of infested plants to make a treatment decision.

If you must continue sampling (seven to 10 plants with a plus), sample five more plants and use the new total number of plants to make a decision.

If no decision is reached, sample additional sets of five plants until 31 plants are sampled. Remember; always use the total number of infested plants to make a decision. If no decision can be made after sampling 31 plants, resample the same field in three to four days.

A treat decision must be confirmed a second time three to four days later. If confirmed, apply and insecticide in three to four days.